Distillation column reboiler control system

ABSTRACT

A control system for regulating the heat input to the reboiler section of a distillation (fractionation) column. Flow-measuring means, for determining the quantity of vapor, passing upwardly from the reboiler section into the fractionation section (stripping and rectification), and internally disposed within the reboiler section, provides a signal which is transmitted to the reboiler heater, thereby adjusting the fuel input thereto in order to regulate the degree of vaporization being effected therein.

United States Patent [I 1 [111 3,881,994 Fickel 1 May 6, 1975 [54]DlSTILLATION COLUMN REBOILER 3,442,767 5/1969 Hall 2031010. 13 CONTROLSYSTEM 3,766,021 10/1973 Randall 202/153 3,803,002 4/1974 Skraba et al202/206 [75] Inventor: R. Gene Fickel, Roselle, 111.

[73] Assignee: Universal Oil Products Company, Primary Examiner -lackSofel' Des Plaines, Ill.

Filed: Apr. 11, 1974 Appl. No.: 459,975

US. Cl. 202/160; 202/206; 202/181; 203/1; 196/132 Int. Cl B0ld 3/42;ClOg 7/00 Field of Search 203/1, DIG. 18; 202/153, 202/160, 206, 181;196/132; 208/D1G. 1

References Cited UNITED STATES PATENTS 12/1965 Kelley et al 203/111/1968 Bellinger 203/1 Raff/nan t I I I I I 1 Attorney, Agent, orFirm-James R. Hoatson, .Ir.; Robert W. Erickson; William H. Page, 11

[57] ABSTRACT A control system for regulating the heat input to thereboiler section of a distillation (fractionation) column.Flow-measuring means, for determining the quantity of vapor, passingupwardly from the reboiler section into the fractionation section(stripping and rectification), and internally disposed within thereboiler section, provides a signal which is transmitted to the reboilerheater, thereby adjusting the fuel input thereto in order to regulatethe degree of vaporization being effected therein.

5 Claims, 1 Drawing Figure Extract/v0 Dish II a lion Column DISTILLATIONCOLUMN REBOILER CONTROL SYSTEM APPLICABILITY OF INVENTION My inventiveconcept, as herein described, encompasses a unique control system, andmethod, for regulating and/or controlling the heat input to the reboilersection of a continuous-flow distillation, or fractionation column. Inthe present specification, as well as the appended claims, the use ofthe term distillation column synonymously alludes to fractionationcolumns", rerun columns", stripping columns, extractive-distillationcolumns," etc. Similarly, for the purposes of describing the presentinvention, the reboiler section" of the distillation column connotesthat portion disposed below the lowermost tray, or deck. That portion ofthe column above the lowermost tray is herein referred to as thefractionation section," and is inclusive of the rectification zone(above the feed tray) and the stripping zone (below the feed tray).

As is recognized by those possessing the requisite expertise in theappropriate art, reboiling a distillation column connotes thecirculation of a hot liquid bottoms fraction from the reboiler sectionthrough an external reboiler heater, wherein at least a portion of theliquid is vaporized; the heated, mixed-phase bottoms material isreturned to the reboiler section. The vapors pass upwardly from thereboiler section into the fractionation section wherein they serve toremove lower-boiling constituents from the liquid phase traversing thecolumn in downward flow. A most important aspect, to be considered forefficient functioning of a distillation column, resides in its thermalbalance and, although many operating variables have an effect uponthermal balance, the effect of heat input via the reboiler section isperhaps the most pronounced. Furthermore, as hereinafter set forth,control of the heat input to the reboiler section is generally the mostdifficult to achieve to the degree required for substantially stablethermal balance.

The foregoing is applicable to reboiler operation considerationsregardless of the precise boiling range of the circulating reboilerbottoms material; however, the difficulties attendant reboiler heatinput control become more pronounced when the reboiler liquid is a purecompound, or exhibits a relatively narrow boiling range i.e. about lF.,or less. Heat input by way of circulating reboiler liquid takes twoforms; (i) the increased sensible heat of the heated liquid passing backinto the reboiler section and, (ii) the latent heat of vaporizationabsorbed by the vapors generated in the external reboiler heater. Thelatter constitutes the source of the greatest quantity of heat input tothe reboiler section and, of necessity, must be subject to close controland- /or regulation. Additionally, the efficiency of separation, toobtain the desired product purity, is largely dependent upon the amountof vapor produced and the control thereof.

Briefly, the control system and method of the present invention isdesigned to achieve a constant rate of vapor production in the reboilerheater by adjusting, or regulating the flow of fuel to the heater inresponse to a signal which is representative of the actual quantity ofvapor passing into the fractionation section of the distillation column.

OBJECTS AND EMBODIMENTS A principal object of the present invention isto provide a method for measuring and controlling the vapors produced inthe reboiler section of a distillation column. A corollary objective isdirected to a control system which is capable of constant heat inputduring steady-state operation.

Further, it is a specific object to afford a control sys tem and method,both of which pennit rapid recovery of a steady-state operation aftersignificant changes in various operating variables.

Therefore, in one embodiment, my invention provides a control system forregulating the heat input to the reboiler section of a distillationcolumn which comprises, in cooperative combination: (a) an inventory ofliquid bottoms material contained in a suitable chamber in said reboilersection; (b) flow-regulating means for passing a portion of said liquidbottoms to a reboiler heater; (c) conduit means for passing heated,mixedphase bottoms material from said heater into said reboiler section;(d) fuel-varying means for adjusting the fuel input to said heater; (e)flow-measuring means for determining the quantity of vapor, in saidmixed-phase bottoms material, passing from said reboiler sectionupwardly into the fractionation section of said distillation column,said flow-measuring means internally disposed within said reboilersection; and, (f) signal-receiving means for establishing a signalrepresentative of the quantity of vapor passing into said fractionationsection and for transmitting said signal to said fuel-varying means,whereby the fuel input to said heater is adjusted in response to thequantity of said vapor; said control system further characterized inthat said reboiler section is partitioned to provide two inventorychambers of said liquid bottoms material, the first of which has saidflow-measuring means disposed therein, and from the second of which saidbottoms material is passed into said reboiler heater.

Other objects and embodiments, encompassed by the present inventiveconcept, will become evident from the following, more detaileddescription. Included is the embodiment which is directed toward amethod for controlling the heat input to the reboiler section of adistillation column, in response to variations in the steady-stateoperation of said column.

SUMMARY OF INVENTION Distillation operations and techniques areextensively employed throughout the petroleum and petrochemicalindustries for the separation and recovery of select fractions of thefeed, or of substantially pure compounds. For example, in a process forthe catalytic reforming a naphtha boiling range charge stocks, thenormally liquid portion of the reaction zone effluent is rerun," oftento provide a light gasoline i.e. boiling range of to 280F. and a heavygasoline i.e. boiling range of 280to 400F. ln adsorption processes,wherein polar hydrocarbons are separated from a mixture thereof withnon-polar hydrocarbons, employing a solvent having greater selectivityfor adsorbing the polar constituents, the ultimately recovered producthas a relatively narrow boiling range, and its distillationcharacteristics are substantially similar to those of pure compounds.Although applicable in both types of distillation techniques, myinvention is most advantageously employed in situations where the columnbottoms fraction, a portion of which serves as the circulating reboilerheating medium, is a pure compound, or a narrow boiling range mixture.Briefly, the basic reboiling technique is designed to provide the amountof vapor ous material required for thermal balance and separa tionefficiency by adjusting the quantity of fuel input to the reboilerheater, in order to regulate the heat input to the reboiler section ofthe distillation column.

It is recognized that the published literature is replete withtechniques designed to control the quantity of heat input to thereboiler section of a distillation column. In view of the voluminousnature of such techniques, no attempt will be made herein to delineateexhaustively the appropriate prior art; a few typical illustrations willsuffice. One popular and ancient prior art technique, now since improvedupon, involves instituting an energy balance around the reboiler heater;a similar scheme computes the energy balance around the reboiler sectionof the column. While affording a small measure of control, bothtechniques entail too many measurements, accompanied by an extremelydifficult, tedious energy balance, and are comparatively inefficient.Other control techniques involve controlling the flow of fuel medium tothe reboiler heater in response either to the temperature of the heatedmaterial return to the reboiler section, or to the rate of mixed-phaseflow. For the latter such method, the quantity of liquid reboilerbottoms material introduced into the reboiler heater must be pre-set byway of flow control means. Flow control of the heated materialreentering the reboiler section suffers from the disability of not beingcapable of precisely measuring vapor flow and depends upon, as anessential element, a constant flow rate to the heater. Temperaturecontrol will generally suffice acceptably in situations where thereboiler liquid has a relatively wide boiling range, but fails miserablywhere it is either a substantially pure compound, or possesses acomparatively narrow boiling range, or where a relatively minor degreeof vaporization is desired.

As hereinbefore set forth, a most important criterion is the measurementof the degree of vaporization in the heated, mixed-phase materialreturning to the reboiler section of the column. This is especiallycritical with respect to substantially pure column bottoms material.Correlations of heat content versus temperature, at given vaporizationpercentages, indicate that a comparatively large delta-T exists per unitof heat content as the percentage vaporization increases in the case ofa liquid bottoms material having a relatively wide boiling range. Thus,a measurable change in temperature will indicate a significant change inthe degree of vaporization, and a corresponding change in the thermalbalance of the column. Such changes can be employed to reset the flow offuel medium to the heater so that more or less liquid will be vaporizedand the Column can be held in close proximity to thermal equilibrium.

However, temperature control in the return conduit is not satisfactorilyeffective when the reboiler bottoms liquid is a substantially purecompound, or one having a narrow boiling range approximately F. or less.The correlations described above indicate that very little (if any)delta-T is available for percent vaporization determination. That is tosay, temperature measurement in any portion of either the reboilerheater circuit, or the lower reboiler section, does not indicateaccurately the degree of vaporization achieved. The temperature willremain virtually the same whether excess vapor, or insufficient vapor,is being generated in the reboiler heater. It becomes, therefore,extremely difficult to maintain the distillation column at or nearthermal equilibrium.

Similarly, when a pure compound, or narrow boiling range componentmixture, is being reboiled, temperature control of the reboiler heaterheat input is not fea sible due to the effect of pressure variationswithin the distillation column. Any shift in column pressure willproduce a corresponding shift in the boiling point of the pure compound,or in the vapor temperature of the narrow boiling range mixture, withoutnoticeably changing the rate or degree of vaporization. Therefore, achange in column pressure will produce a temperature fluctuation whichis not truly indicative of a change in the reboiling function.Accordingly, a temperature control system will effect a compensatingchange in heat input when no such compensation is required.

The control system of the present invention overcomes the deficienciesof prior art techniques, especially with respect to substantially purecompounds, or narrow boiling component mixtures. This is accomplishedthrough the use of a novel reboiler section design which permits direct,internal measurement of the flow of generated vapor upwardly from thereboiler section to the fractionation section. A signal, representativeof the quantity of vapor passing into the fractionation section istransmitted to fuel-varying means for adjustment, or regulation of thefuel input to the reboiler heater. Thus, there is afforded a constantrate of vaporized material, passing into the fractionation section, fora steady-state status of the various operating variables, and a controlsystem which readily and rapidly responds to compensate for variationsin the steadystate operation, such that desired separation efficiency issubstantially unaffected. An additional advantage resides in the factthat the lowest percentage vaporization, for a given set of operatingvariables, is capable of being maintained. The internal measurement ofthe flow of vapors is extremely accurate and sensitive since it isaccomplished within the reboiler section in a substantially liquid-freeenvironment.

Examples of processes, wherein the separation and recovery of a purecompound, or narrow boiling range mixture forms an integral part, and towhich the pres ent invention may be advantageously applied, include, butnot by way of limitation: (i) the recovery of styrene from anethylbenzene dehydrogenation system; (ii) the separation of one xyleneisomer from a mixture thereof with other xylene isomers; (iii) aromatichydrocarbon separation from a mixture thereof with non-aromatichydrocarbons; and, (iv) the separation and recovery of ethylbenzene froma mixture thereof with various xylene isomers, etc. The particular use,to which the present invention is put, is not to be considered a featurelimiting upon the scope and spirit thereof as defined by the appendedclaims.

For the purpose of additional illustration, as well as the descriptionof the accompanying drawing, further discussion will be restricted tothe integration of the present reboiler control system into a processfor the selective extraction of aromatic hydrocarbons from a mixturethereof with non-aromatic hydrocarbons including both paraffins andnaphthenes. One such process involves extractive distillation of themixed hydrocarbonaceous feed stream with a water-soluble solventselective for the adsorption of aromatic hydrocarbons e.g. asulfolane-type solvent. Extractive distillation conditions include asolvent water content of about 0.5 to 20.0 percent by weight, a solventto hydrocarbon feed ratio of about 2.0: 1.0 to 60:10, a distillationcolumn pressure ranging from 90 mm. Hg, absolute, to about 40.0 psig.,an overhead temperature from 130 to about 330F. and a reboiler bottomstemperature from 170 to about 355F. There is provided a liquid extractbottoms stream relatively free from nonaromatics and comprising solventand aromatic hydrocarbons, and an overhead vaporous raffinate comprisingnon-aromatics, water (as steam) and a relatively minor quantity of thesulfolane solvent. The raffinate is condensed and water-washed torecover substantially solvent-free non-aromatic hydrocarbons containedin the extract phase are recovered in a solvent recovery column of thevariety well known and thoroughly described in the prior art. Steam isemployed as a stripping medium to separate aromatic hydrocarbons fromthe sulfolane solvent. An overhead product of aromatics and steam,substantially free from solvent, is condensed to recover the finalextract product. The water is generally returned to the raffinatewater-wash column. Aromatic hydrocarbon recovery generally exceeds 96.5percent by volume, based upon the charge stock, and the aromatic purityis greater than 99.0 percent.

BRIEF DESCRIPTION OF DRAWING In further describing the present controlsystem and its method of operation, reference will be made to theaccompanying drawing. It is understood that the drawing is presentedsolely for illustration purposes, and the same is not intended to beconstrued as limiting upon the scope and spirit of my invention asdefined by the appended claims. Therefore, miscellaneous appurtenances,not required for an understanding of the inventive concept, have beeneliminated, or reduced in number. Such items are well within the purviewof one possessing skill in the art. Presented in the drawing is anextractive distillation column 1 which is typically employed in thesolvent extraction of aromatic hydrocarbons, utilizing sulfolane ashereinabove described. Extractive distillation column I is shown ashaving a fractionation section 4 located above the lowermost tray 6, anda reboiler section 4a located below tray 6. Reboiler section 4a ispartitioned by chordal baffle 9 to provide two liquid inventory chambers9a and 9b.

DETAILED DESCRIPTION OF DRAWING With particular reference now to thedrawing, the control system and method will be described in conjunctionwith a commercially-scaled unit designed to process 6,723 Bbl/day(863.04 mols/hr.) ofa heptaneplus fraction obtained from an attendantcatalytic reforming unit. The charge stock consits of 83.96 percent byvolume of aromatic hydrocarbons, 2.45 percent of naphthenes and l3.59percent paraffins. The solvent is sulfolane (2,515.5 mols/hr.)containing 3.25 percent water, and enters the upper portion of column 1,via line 3, at a temperature of about 260F. The solvent to feed molratio approximates 30:10, and the hydrocarbon feed is introduced vialine 2. Operating pressures include a reboiler section pressure of aboutl7.0 psig., a pressure of about 12.0 psig. at the locus of hydrocarbonfeed and a top pressure of about 7.0 psig. The reboiler sectiontemperature is about 350F. and the vaporous raffinate overhead stream,in line 5, is at a temperature of about 285F.

Distillation column 1 is shown as having an upper fractionation section4 which, for the purposes of the present illustration, includes alltrays above tray 6, or both the stripping and rectification sections.That portion of column 1 below tray 6 is herein referred to as thereboiler section. Located entirely within reboiler section 4a, isflow-measuring means generally indicated as 7. Flow-measuring means 7 isformed, in part, by chordal baffle 9 which terminates a finite distancebelow tray 6, and extends downwardly through reboiler section 4a, beingimmovably connected to the internal surface of the bottom head ofcolumn 1. There is provided, thereby, two liquid inventory chambers and9b, and, in combination with partition 10, there is formed a riser 11.In this illustration, an orifice plate is placed in riser 11 whichsupplies the flow-measuring means to determine the quantity of vaporousmaterial passing therethrough into fractionation section 4. A riser cap8 is also provided in order to prevent liquid material from tray 6 fromentering riser-orifice 11. Although illustrated and referred to hereinas an orifice, the flow-measuring means can take the form of a venturi.The essential feature resides in the internal measurement of vapor flowinto the fractionation section, and preferably in a substantiallyliquid-free environment. Thus, all the liquid material passingdownwardly from tray 6 is collected in liquid inventory chamber 9b.

During the start-up of the extractive distillation process, all theillustrated controls are set, and initial operating variables areattained manually, to achieve ther mal equilibrium and a substantiallysteady-state operation in accordance with the desired separationefficiency. Flow-Recorder-Controller (FRC) 13, which receives a signal,via line 12, representative of the quantity of vapors passing upwardlythrough riser-orifice 1], is pre-set to transmit the signal via line 14to fuelvarying means 15.

For a given quantity of charge stock, at a constant composition, FRC 13is set to provide the minimum quantity of vapors, passing upwardly intofractionation section 40. If an insufficient quantity of strippingvapors are supplied by reboiler heater l7, non-aromatic hydrocarbonswill appear in the liquid bottoms product; conversely, an excessquantity of vapors will, in effect, throw" solvent and/or aromatics intothe nonaromatic raffinate overhead. In either situation, the resultingupset in thermal balance adversely affects the desired separationefficiency of the distillation column. Only through the use of thepresent invention, wherein the vapor flow is internally measured, cancontrolled, steady-state operation be maintained. Fluctuations inoperating variables, which ultimately manifest an adverse affect withrespect to themial balance, result primarily from: (i) changes in feedcomposition (aromatic/non-aromatic ratio); (ii) varying hydrocarboncharge stock rate; and, (iii) a change in the solvent- /feed mol ratio.Certainly, other operating variables, such as temperature, pressure,reflux rate (if any), water content of the solvent, desired productstate, etc., affect thermal balance, but not to as great a degree.

Control valve 15, in line 16, regulates the fuel input to reboilerheater 17, in response to the signal being transmitted from FRC 13 vialine 14. Reboiler liquid is withdrawn from inventory chamber 9b inresponse to Level-Recorder-Controller (LRC) 28 which senses the level ofliquid bottoms therein via conduits 29 and 30. LRC 28 transmits a signalto FRC 32 via line 31, reset the set-point thereof. FRC 32 senses, vialine 33 and orifice 36, the flow of liquid through conduit 37, and makesthe necessary adjustment in control valve 35. The amount of liquidbottoms material, at a temperature approximating 3 l2F., flowing throughconduit 37 into heater 17, is about 4,381 mols/hr. Sufficient fuel issupplied to heater 17, through line 16, to produce a heated mixed-phasefluid, in line 38, having a temperature of about 350F. The mixed-phaseis reintroduced into reboiler section 4a through inlet port 39. Ininventory chamber 90, a phase separation takes place to the extent that1,164 mols/hr. of vapor pass upwardly ough the riser-orifice 11, intothe fractionation section. The 3,217 mols/hr. ofliquid are withdrawn byway of conduit 27, and transported thereby to a solvent re covery systemnot illustrated in the drawing.

Level-lndicating-Controller l8 senses the liquid level in inventorychamber 9a by way of conduits l9 and 20. Its principal function is tomaintain a liquid seal at the bottom of the inventory chamber, whilesimultaneously maintaining the liquid level out of contact with theriser-orifice. A signal is transmitted by way of line 21 to FRC 22, tore-set the set point thereof. FRC 22 senses the flow ofliquid throughline 27 via conduit 25 and orifice 26, and accordingly adjusts, via line23, the flow through control valve 24.

in the present illustration, the steady'state operation thus fardescribed results in an overhead raffinate stream. transported to awater-wash column via line 5, in the amount of about l59.8 mols/hr. ofwhich 1.25 percent is sulfolane solvent and l3.52 percent is aromatichydrocarbons. The stream in line 27, being transported to the solventrecovery system, is in an amount of 3,720.7 mols/hr., of which 2,515.5mols/hr. are solvent. Following removal of solvent and water,, there isrecovered an aromatic-rich product stream, in the amount of 705.2mols/hn, of which only 0.31 percent by volume constitute non-aromatichydrocarbons.

From the foregoing, it will be noted that 97.02 percent of the feedaromatics are recovered (on a oncethrough basis), and that the puritythereof exceeds 99.0 percent. The control system of my inventionmeasures and maintains a steady stripping vapor rate to thefractionation section. As hereinbefore set forth, any one or acombination of operating variable fluctuations can cause an upset in thethermal balance of the column, and an accompanying upset in the desiredseparation efficiency. Regardless of the precise cause, or causes, itwill be presumed that the ultimate effect is an increase in the flow ofliquid material downwardly in the fractionation section. Since riser cap8 effectively channels all the liquid into inventory chamber 9b, theliquid level therein will cause LRC 28 to cascade a reset signal to FRC32 which, in turn, effects an opening of control valve 35.

The increased rate of liquid flow through conduit 37 into reboilerheater l7, effectively decreases the degree of vaporization beingeffected therein. As previously stated, the resulting heated mixed-phasein conduit 38 is introduced into the second inventory chamber 90, andFRC l3 senses the rate of vapor flow through riserorifice 1 l. Therepresentative signal is, of course, to the effect that the vapor rateupwardly in the fractionation section is lower than desired. This signalis transmitted by FRC' 13, through line 14, to heat-varying means 15,and increases the fuel input to heater 17, via line 16. As a result, therate of vaporization is increased and the quantity of vapors passingthrough riser-orifice l1 correspondingly increases.

Simultaneously with the foregoing, LIC 18 senses an increasing liquidlevel in inventory chamber 9a. The resulting transmitted signal causescontrol valve 24 to open wider, thereby sending the additional liquidbottoms to the solvent recovery facility. It is understood that thecascade-type control loops represented (l by LRC 28, FRC 32 and controlvalve 35, and (2) MC 18, FRC' 22 and control valve 24, are not essentialto my invention. They are illustrated as preferred instrumentationtechniques to achieve smoother functioning in the operation thereofLikewise, reboiler heater 17 may take the form of a tube and shellheat-exchanger, or a direct-fired heater as illustrated.

The foregoing is believed to convey the essence of the control system ofthe present invention and its method of operation. Advantages attendantits integration into fractionation and/or distillation facilities willbe evident to those having expertise in the art.

I claim:

1. A control system for regulating the heat input to the reboilersection ofa distillation column which comprises, in cooperativecombination:

a. a chamber in said reboiler section for receiving liq uid bottomsmaterial from the fractionation section of said column;

b. a reboiler heater having feed input means thereto connected to saidchamber, flow regulating means for controlling the flow of said liquidbottoms from said chamber to said reboiler heater in accordance with theliquid level in said chamber;

0. conduit means in interconnection with said reboiler heater forpassing heated, mixed'phase bottoms material from said heater into apartitioned and segregated portion of said chamber in said reboilersection;

d. fuel-varying means in a fuel input means in interconnection with saidheater for adjusting the fuel input to said heater;

e. vapor flow-measuring means in said reboiler section responsive to thetotal quantity of upward flowing vapor from said mixed-phase bottoms material passing from said segregated portion of said reboiler sectionupwardly into the fractionation section of said distillation column,said flow measuring means being internally disposed within saidsegregated portion of said reboiler section; and

f. signal-generating means in said segregated portion of said reboilersection for establishing a signal representative of the volumetric flowrate of vapor from said segregated portion passing into saidfractionation section, and signal transmitting means for modifying andtransmitting the resulting signal to said fuel varying means, wherebyfuel input through said fuel input means to said heater is decreased inresponse to increasing flow rates of said vapor and increased inresponse to decreasing flow rates of said vapor; said control systembeing fur ther characterized in that said reboiler section ispartitioned to provide two inventory chambers of said liquid bottomsmaterial, the first of which is said segregated portion which has saidvapor flow measuring means disposed therein and the second of which isconnected to said freed input means.

2. The control system of claim 1 further characterized in that a secondflow-regulating means effects withdrawal of bottoms material from thefirst of said inventory chambers and out of said distillation column.

3. The control system of claim 2 further characterized in that saidsecond flow-regulating means, includes means to maintain the level ofthe liquid bottoms in said first inventory chamber out of contact withsaid flow-measuring means.

4. A method for controlling the heat input to the reboiler section of adistillation column, in response to variations in the steady-stateoperation of said column, which method comprises the steps of:

a. regulating the quantity of liquid bottoms material withdrawn from oneside of a partitioned reboiler section in response to the liquid levelin said one side and introducing same into a reboiler heater;

b. passing heated, mixed-phase bottoms material from said heater intothe second side of said partitioned reboiler section;

' the fuel input with reduced vapor flow and vice versa; and,

e. withdrawing excess liquid bottoms material out of said distillationcolumn from the second side of l said partitioned reboiler section inresponse to the liquid level in said second side maintained below thepoint of measuring vapor flow.

5. The method of claim 4 further characterized in that the quantity ofvapor, in the heated, mixed-phase bottoms material from said heater,which passes into said fractionation section, is measured within saidreboiler section in a substantially liquid-free atmosphere. t

1. A control system for regulating the heat input to the reboiler section of a distillation column which comprises, in cooperative combination: a. a chamber in said reboiler section for receiving liquid bottoms material from the fractionation section of said column; b. a reboiler heater having feed input means thereto connected to said chamber, flow regulating means for controlling the flow of said liquid bottoms from said chamber to said reboiler heater in accordance with the liquid level in said chamber; c. conduit means in interconnection with said reboiler heater for passing heated, mixed-phase bottoms material from said heater into a paRtitioned and segregated portion of said chamber in said reboiler section; d. fuel-varying means in a fuel input means in interconnection with said heater for adjusting the fuel input to said heater; e. vapor flow-measuring means in said reboiler section responsive to the total quantity of upward flowing vapor from said mixed-phase bottoms material passing from said segregated portion of said reboiler section upwardly into the fractionation section of said distillation column, said flow measuring means being internally disposed within said segregated portion of said reboiler section; and f. signal-generating means in said segregated portion of said reboiler section for establishing a signal representative of the volumetric flow rate of vapor from said segregated portion passing into said fractionation section, and signal transmitting means for modifying and transmitting the resulting signal to said fuel varying means, whereby fuel input through said fuel input means to said heater is decreased in response to increasing flow rates of said vapor and increased in response to decreasing flow rates of said vapor; said control system being further characterized in that said reboiler section is partitioned to provide two inventory chambers of said liquid bottoms material, the first of which is said segregated portion which has said vapor flow measuring means disposed therein and the second of which is connected to said freed input means.
 2. The control system of claim 1 further characterized in that a second flow-regulating means effects withdrawal of bottoms material from the first of said inventory chambers and out of said distillation column.
 3. The control system of claim 2 further characterized in that said second flow-regulating means, includes means to maintain the level of the liquid bottoms in said first inventory chamber out of contact with said flow-measuring means.
 4. A method for controlling the heat input to the reboiler section of a distillation column, in response to variations in the steady-state operation of said column, which method comprises the steps of: a. regulating the quantity of liquid bottoms material withdrawn from one side of a partitioned reboiler section in response to the liquid level in said one side and introducing same into a reboiler heater; b. passing heated, mixed-phase bottoms material from said heater into the second side of said partitioned reboiler section; c. measuring within said second side of said reboiler section the quantity of vapor in said heated, mixed-phase bottoms, which passes from said second side of said reboiler section into the fractionation section of said distillation column; d. regulating the fuel input to said reboiler heater in response to a signal representative of the said measured quantity of vapor flowing into said fractionation section, said regulation comprising reducing the fuel input with reduced vapor flow and vice versa; and, e. withdrawing excess liquid bottoms material out of said distillation column from the second side of said partitioned reboiler section in response to the liquid level in said second side maintained below the point of measuring vapor flow.
 5. The method of claim 4 further characterized in that the quantity of vapor, in the heated, mixed-phase bottoms material from said heater, which passes into said fractionation section, is measured within said reboiler section in a substantially liquid-free atmosphere. 